This is the second, validation phase, of a multi-institutional multidisciplinary classification study of 400 preschool children with developmental language disorders (dysphasia or DLD), autistic spectrum disorders (ASD), and/or mental deficiency without autism (NAMD). During the first 3 years of the project, the children were studied using a common battery of tests of neurologic status, neuropsychology, language, sociability, and play. The data were entered into a common data base and submitted to multivariate analyses, including clustering. A clinical-empirical model was used to (1) define the best measures to recreate these 3 clinical groups; and (2) subtype children with dysphasia and ASD. This internally valid typology will now be validated by (1) longitudinal study of the 400 children in the original cohort at ages 7 and 9 y. to determine subtype stability and outcome; (2) cross-validation in a new sample of 800 children screened by independent investigators at ages 3-5y. for the presence of DLD, ASD, and NAMD; 150 with DLD, 150 with ASD, and 100 with NAMD will be subtyped using a scaled-down battery; and with biologic measures including (3) electrophysiologic recording of event-related potentials and their mapping on the scalp in prototypical DLD and ASD children, compared to normals, while they are engaged in linguistic tasks; (4) morphometric neuroimaging of the brain using MRI in prototypical and control children to seek anatomic correlates of these subtypes; and (5) genetic study of the 800 children in the longitudinal and cross-validation cohorts using pedigree analysis, chromosome studies, and genetic linkage studies of large families with multiple affected members. The product of this study will be an internally, externally, and cross-validated typology of the disorders of language, sociability, and cognition in preschool children. The study will specify the best measures for professionals from different disciplines to arrive at a common diagnosis. It will enhance communication among investigators and clinicians by fostering a common terminology, clarify diagnosis and prognosis, and define the etiology and neurologic basis for some subtypes of these disorders. It will promote rational intervention by defining the deficits underlying particular subtypes of DLD and ASD.